Uploading post thumbnail leads to PHP Fatal error

Added thumbnail to the post_fields parameter on the shortcode but uploading an image leads to the following fatal error:

[12-Oct-2017 15:21:39 UTC] PHP Notice:  Undefined index: storage in /Users/florian/htdocs/nsm/web/wp-content/themes/nsm/vendor/meta-box/inc/field.php on line 290
[12-Oct-2017 15:21:39 UTC] PHP Fatal error:  Uncaught Error: Call to a member function add() on null in /Users/florian/htdocs/nsm/web/wp-content/themes/nsm/vendor/meta-box/inc/field.php:319
Stack trace:
#0 /Users/florian/htdocs/nsm/web/wp-content/themes/nsm/vendor/meta-box/inc/field.php(539): RWMB_Field::save(Array, Array, 62, Array)
#1 /Users/florian/htdocs/nsm/web/wp-content/themes/nsm/vendor/mb-frontend-submission/inc/class-mb-frontend-post.php(154): RWMB_Field::call(Array, 'save', Array, Array, 62)
#2 /Users/florian/htdocs/nsm/web/wp-content/themes/nsm/vendor/mb-frontend-submission/inc/class-mb-frontend-post.php(74): MB_Frontend_Post->save_thumbnail()
#3 /Users/florian/htdocs/nsm/web/wp-content/themes/nsm/vendor/mb-frontend-submission/inc/class-mb-frontend-form.php(82): MB_Frontend_Post->save()
#4 /Users/florian/htdocs/nsm/web/wp-content/themes/nsm/vendor/mb-frontend-submission/inc/class-mb-frontend-form-shortcode.php(62): MB in /Users/florian/htdocs/nsm/web/wp-content/themes/nsm/vendor/meta-box/inc/field.php on line 319

User is logged in. Happens to admin as well as subscriber level.